The following are instructions for a more somatic metta practice. Maybe someone will find it an easier way to get into metta.
Method 1: This is a samatha(tranquility) meditation, and as such, it is best to have the mind unified upon a single phrase. This might be “May I be happy”, “May I be happy and calm”, “May I be loved and loving”. Whatever works for you. The idea is just that the phrase resonates with your own deepest intentions such that the mental impression of the words evoke pleasurable feelings in the body. It may be useful to familiarize oneself with the location and sensations associated with certain feeling words. This step is not necessary, but may be useful in the beginning. To do this, recall a time you experienced deep joy, happiness, or contentment, or simply imagine the experience of such feelings. When the pleasurable feeling is evoked get a sense of where this is located in the body and the associated sensations.
Next, feel what it is to experience life as the body. Let go of thought and drop into this vivid pattern of sensations. Do this until the mind is relatively calm and the experience of the body predominates. Then begin reciting the phrase of choice, accept rather than “May I be happy”, try using the phrase “May this body be happy”, or “May the body be happy and calm”. Saying the phrases in such a way draws one’s attention towards the experience of the body. When the intensity of pleasant sensations begins to rise, feel into the sensations after each phrase, sustaining awareness on the body or the phrases throughout. If difficult feelings arise, feel into them in the body. Keeping your attention on the area of the body where the tightening is felt most clearly, practice saying the phrases and directing the energy towards this physical location.
Method 2: This is the simpler method, and more closely related to typical breath meditation. Put your attention on the heart center, the area in the center of the chest. Spend five or ten minutes experiencing the sensations in this area as a unified whole. When your attention drifts, return it to this area. After several minutes begin dropping the feeling word from each of three metta phrases at the beginning of the out breath. For example, putting one’s attention on the in breath at the center of the chest, at the beginning of the exhale imagine you are sending the energy of the word happy, or peaceful, or loved, right into the center of the chest, alternating between the three at the beginning of each out breath. After saying the word let the attention rest on the center of the chest and the associated feelings that the word conjures up. This practice can lead to deep, pleasurable states of unification, as well as the nourishing qualities of metta.